Cartwheel Coronal Mass Ejection

May
27, 2008: Imagine a billion-ton cloud of gas launching
itself off the surface of the sun and then ... doing a cartwheel.
That's exactly what happened on April 9, 2008, when a coronal
mass ejection or "CME" pirouetted over the sun's
limb in full view of an international fleet of spacecraft.
Even veteran solar physicists were amazed.

But
that's not all. While one part of the cloud did a cartwheel,
another part did a backflip at the same time. As strange as
it sounds, this could be the normal way solar explosions unfold,
say researchers analyzing the data.

"What
a rare and exciting observation," says Ed DeLuca of the
Harvard-Smithsonian Center for Astrophysics (CfA) who announced
the findings at the American Geophysical Union meeting in
Fort Lauderdale, Florida, on May 27th. "It is showing
us the secret inner workings of a process called 'magnetic
reconnection' central to solar flares and CMEs."

A
picture is worth a thousand words. Click on the image to set
the scene in motion:

Above:
A solar explosion spinning two ways at once. The cartwheel
is particularly vivid in a high-contrast XRT movie: play
it!

These
videos reveal a billion tons of hot, magnetized gas twirling
at speeds in excess of 1000 km/s. The cartwheel (left; recorded
by the X-Ray Telescope onboard Japan's Hinode spacecraft)
spins one way while the backflip (right; recorded by UV cameras
onboard NASA's TRACE spacecraft) spins the other.

How
can an explosion spin in two directions at once?

DeLuca
explains: "We think we are seeing a twisted 'flux tube'
of solar magnetism unfurl. One end of the tube spins clockwise,
the other counterclockwise." This unfurling action pumps
energy into the explosion, heating the CME and propelling
it away from the sun.

To
better understand the process, rummage through your desk and
pull out a rubber band. Hold one side of the loop between the
thumb and forefinger of your right hand; hold the other side
of the loop with your left hand. Stretch the rubber band taut
and start twisting (roll the rubber band between thumb and forefinger).
The band becomes tight and knotted and filled with latent energy.
Keep twisting, if you dare, until—crack!—the band ruptures,
snapping back on your fingertips and making a nasty welt.

You've
just simulated a solar flare at your desktop.

Magnetic
flux tubes on the sun behave a lot like rubber bands, researchers
believe. They get twisted and knotted and filled with latent
energy, until—crack!—the field lines rupture, producing an
explosion more powerful than a hundred million hydrogen bombs.
Remember the rubber band untwisting as it hurtled back toward
your fingertips? There you have the cartwheel and backflip,
writ small.

The
CME, however, was merely the beginning. "The really interesting
developments came later," says solar physicist Leon Golub
of the CfA. Hours after the initial blast, the ruptured magnetic
flux tube began to heal itself. Rubber bands never do this
trick, but magnetic fields do because, basically, Nature abhors
a broken flux tube. Thanks to the high-resolution of Hinode's
X-Ray Telescope, says Golub, "we have witnessed a phase
of magnetic reconnection never before seen in such detail."

The
healing process began with the formation of a tall X-ray spike
jutting out of the blast site. "This is a current sheet
seen edge-on," says Golub. Current sheets are where magnetic
fields of opposite polarity meet and rejoin. Hinode's X-ray
movie shows material left behind by the CME flowing back down
into the region from above: click
to play. The current sheet seems to guide the flow as
the area reloads for possible future explosions.

Right:
An X-ray snapshot of the post-eruption current sheet. Credit:
Hinode XRT.

Spacecraft
have recorded thousands of CMEs before, but this CME is giving
up its secrets more readily than the others. Co-investigator
Kathy Reeves of the CfA explains why:

"We
were lucky. The solar flare was hidden just behind the limb
of the sun; this eliminated the blinding flash so that Hinode
could take long exposures of the fainter CME above."

How
significant are the data? The CfA researchers are planning
an entire workshop dedicated to the study of this one CME.
They and others will bring together data from a fleet of spaceships
including Hinode, TRACE, SOHO, STEREO and RHESSI to gain a
more complete understanding of solar eruptions.

Their
conclusions will go far beyond the sun, however. Magnetic
reconnection is a process fundamental to many realms of astrophysics.
"It happens in black holes, pulsars, active galactic
nuclei, planetary magnetospheres—you name it," says DeLuca.
"The sun is a great big laboratory where we can watch
it happen."

And
who wouldn't want to watch a billion-ton cartwheel? Stay tuned
to Science@NASA for further developments.

Led
by the Japan Aerospace Exploration Agency (JAXA), Hinode
is a collaborative mission that also includes the space
agencies of the United States, Great Britain and Europe.
Ed DeLuca of the Harvard-Smithsonian Center for Astrophysics
(CfA) is principal investigator for Hinode's X-Ray Telescope,
which was developed at the CfA. The Marshall Space Flight
Center managed the NASA instrument component integration
for NASA Headquarters, is managing the science operations
for NASA and is also supporting science operations in
Japan.